The present invention relates generally to an exposure apparatus, and more particularly to a life evaluation of a light source used in an exposure apparatus for exposing an object, such as a single crystal substrate of a semiconductor wafer etc. and a glass plate for a liquid crystal display (“LCD”). The present invention is suitable, for example, for an exposure apparatus that uses an extreme ultraviolet (“EUV”) light as a light source for exposure.
Conventionally, the photolithography technology has employed a reduction projection exposure apparatus using a projection optical system to project a circuit pattern of a mask (reticle) onto a wafer, etc., in manufacturing fine semiconductor devices such as a semiconductor memory and a logic circuit.
The minimum critical dimension to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution. Thus, along with recent demands for finer semiconductor devices, uses of shorter ultraviolet light wavelengths have been promoted—from an ultra-high pressure mercury lamp (I-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
However, the lithography using the ultraviolet light has limitations when it comes to satisfying the rapidly promoted fine processing of a semiconductor device. Therefore, a reduction projection optical system using the EUV light with a wavelength of 10 to 15 nm shorter than that of the ultraviolet light (referred to as an “EUV exposure apparatus” hereinafter) has been developed to efficiently transfer very fine circuit patterns of 0.1 μm or less. The EUV exposure apparatus typically uses a light source (LPP light source) using a laser produced plasma (“LPP”) manner and a light source (DPP light source) using a discharge produced plasma (“DPP”) manner.
The LPP light source irradiates a highly intensified pulse laser beam to a target material (such as a metallic thin film, inert gas, and droplets) supplied to a vacuum chamber by a gas jet etc., thus generating high-temperature plasma for use as EUV light with a wavelength of about 13.4 nm emitted from the plasma. On the other hands, the DPP light source applies high-voltage between electrodes, emits gas such as xenon for discharge, induces the high-temperature plasma, and generates the EUV light. A condenser mirror that condenses the EUV light from the plasma is provided in these EUV light sources to efficiently use the EUV light.
However, the LLP light source generates not only the EUV light from the target material but also flying particles called debris. A supply mechanism that supplies the target material also emits the debris. Moreover, the electrode emits the debris in the DPP light source. The debris causes contaminations, damages, and lowered reflectivity of optical elements, making uneven the light intensity and deteriorating the throughput. Accordingly, emitting inert gas such as helium (He) as a buffer gas, and a debris mitigation system is provided to remove the debris.
On the other hands, while the EUV light source repeats emissions of the EUV light, the light intensity of the EUV light irradiated from the EUV light source deteriorates by damages and deterioration of the electrode, lowered reflectivity of mirrors caused by flying particles such as debris that is not removed. In this case, it is necessary to exchange the EUV light and each member in the EUV light source. Then, a life evaluation of the EUV light source or a life evaluation of each member in the EUV light source has been proposed. See, for example, Japanese Patent Application, Publication No. 2003-224053. The EUV exposure apparatus disclosed in Japanese Patent Application, Publication No. 2003-224053 measures the light intensity of the EUV light, and controls an exposure dose or evaluates mirror damages based on the measured result. The light intensity of the EUV light is measured by detecting a part of an illumination light diverged in a position of a reflective integrator using a detector.
However, Japanese Patent Application, Publication No. 2003-224053 directly (without through a condensing point) detects the EUV light irradiated from the EUV light source, and does not provide the detector in a clean environment (high vacuum atmosphere). Therefore, the detector used deteriorates, and does not provide correct information. In other words, because the detector is provided in a space that generates the plasma, the detector is subject to the flying particles such as debris, and cannot detect the light intensity of the EUV light.
In the LPP light source, the detector that is provided in the same space as the plasma (target supply mechanism) and the electrode is subject to noise and electromagnetic wave by high-frequency noises from the target supply mechanism that supplies the target with a high-speed frequency using a piezoelectric element etc. In the DPP light source, the detector that is provided in the same space as the plasma (target supply mechanism) and the electrode is subject to noises and electromagnetic waves by high-frequency noises generated by the high voltage applied between electrodes. Therefore, the detected result by the detector includes an error, or cannot provide correct information of the EUV light, and the life of the EUV light source cannot be evaluated correctly.
The erroneous life evaluation of the EUV light source would cause exposure to continue with the EUV light source that must be exchanged or the EUV light source that does not need to be exchanged to be improperly exchanged, thereby deteriorating the exposure performance such as an imaging performance, cost, and throughput.